I recall that this is because these rips are quieter than normal CDs when ripped with the HDCD DSP. By the way, once your rip with the HDCD DSP and create the 24/44.1 files, you don't need an HDCD capable player. That's the point of the DSP. It produces an approximation of what the CD would sound like without needing the HDCD capable player. You might also want to look at:
https://forum.dbpoweramp.com/showthr...Best-practices
p.s. Many CDs that are labeled as HDCD don't actually use the features of HDCD that benefit from a capable player or ripping using the HDCD DSP. That's because they don't use the Peak Extension feature. Grateful Dead HDCDs after about 2012/13 are examples of this.
EDIT: I don't think you need to rerip these files. Just use dbpa Batch Converter on these albums and include the HDCD DSP in the conversion.